ZA200608502B - Method and device for transmitting a signal in a multiantenna system, signal, and method for estimating the corresponding transmission channels - Google Patents

Description

Method and device fom transmitting a signal in a mmulti-antenna system, signal and method for estimating corresponding transmission chamnnels . 1. Field of the Invention

The field of the invention is that of digital communicatiors by wireless.

More specifically, the invention rela tes to the transmission and reception, and especially the estimation of transmission channels in a MIMO ("Multiple Input

Multiple Output") type or MISO ("Multiple Input Single Output") type multiple- antenna swstem, through the transmission of signals subjected to a space-time and/or spaece-frequency encoding.

Mowre specifically again, the invention can be applied to multi-antenna systems irmplementing several transrmit antennas, in particular nore than two transmit artennas. The signals comprise reference symbols, known_ to at least one receiver and enabling this receiver to estimate the transmisssion channels corresponAing to each of the transmit zantennas.

An example of an applicatiom of the invention is in the field of radio communic- ations, especially for third, fourth and subsequent generation systems.

The invention can be applied to uplink communications (from a terminal to a base s tation) as well as to downlimnk communications (from a b ase station to a terminal). 2. Prior art solutions

There are several and knowr techniques of estimation of transmission channels ira multi-antenna system co mprising several transmit antennas.

Most of these estimation techniques are limited to the application of a space-time= encoding or space-frequesncy encoding in OFDM-typee multi-carrier systems.

Thus, the first systems proposed all used orthogonal space-time block codes.

Alaamouti in "A Simple Transmit Diversity Techniques for Wireless

Communications”, IEEE Journal on Selected Areas in Communications, pp. 311- 335, vol. 65, 1998, presented the first system using an orthogonal space-time block code of rate 1 (where rate is «defined as the ratio between thme number N of symbols transmitted and the number IL of symbol times during whiech they are transmitted) for two transmit antennas. . A major drawback off Alamouti's orthogonal space—time codes is that they 5S are limited to two-transmit-zantenna systems and that it is- not possible to extend their use directly to a system. with more than two transmit antennas while keeping an unitary rate,

Tarokh and al. ("Speace-time Block Codes from Orthogonal Designs",

IEEE Transactions on Information Theory, 1999, 45, (5D, pp. 1456-1467) then extended the orthogonal space-time block codes to systerms comprising three or four transmit antennas. However, the rates R=N/L obtained were only 1/2 or 3/4.

One drawback of Tamokh's orthogonal space-time codes therefore is that, although they are adapted to- systems implementing a greater number of transmit antennas (three or four antenmas), they have a rate of less thaan 1.

Barhumi and al. in "Pilot Tone-based Channel estimation for OFDM

Systemes with Transmitter D®iversity in Mobile Wireless C hannels" then proposed a channel estimation techraique for multi-antenna OFDM (SISO-OFDM or

MIMO-OFDM) systems relying on a classic OFDM charnel estimation system, implementing an extinction of certain carriers. However,. one drawback of this estimation technique in a MIMO system is that the insertican of reference symbols generally causes major loss of spectral efficiency whenever, for each transmit antenna, a reference symbol is transmitted on a reference carrier at a given point in time while no data whatsoever is transmitted on the oth _er carrier or carriers so as not to disturb the estimatiosn of the transmission channel.

Other research was subsequently conducted by Fragouli and al. in "Training Based Channel Estimation for Multiple-Antenna Broadband

Transmissions" on the learnirg sequences that can be used for channel estimation for multi-antenna systems.

Subsequently, Stirlin g-Gallacher and al. ("Improving performance of 30» coherent coded OFDM systems using space time transmit diversity", Electronics

Letters , Vol. 37 N, March 2«01, "Practical Pilot Patterns -coherent coded OFDM systems using space time transmit diversity", European Wireless 20-02 conference, 25-28 February 2002, Florence) envisaged. a channel estimation technique for

MIMO-OFDM systems, restricted to twwo-transmit-antenna ssystems using . orthogonal spaace-time codes of the Alamout® or Tarokh type.

One drawback of this estimation techhnique is that the number of transmit antennas of th e transmission system is limisted by the use of prior—art orthogonal space-time blosck codes.

Thus, according to the prior-art techniques, there are= no complex orthogonal un-it-rate codes for systems havi ng more than two trarmsmit antennas.

This diminishes spectral efficiency. 3. Goals of the invention

The in-vention is aimed especially a® overcoming these dramwbacks of the prior art.

More specifically, it is a goal of the- invention to provide & technique for the estimation of transmission channels in = multi-antenna system_ implementing more than two transmit antennas.

It is another goal of the invention to propose a technique of this kind that is more efficient and performs better than tke prior-art techniques- while having lower complexity.

It is sset another goal of the inwvention to provide a technique of transmission o fa signal comprising reference symbols implementirag a space-time encoding and/or space-frequency encoding rmatrix. In particular, it Hs a goal of the invention to provide a unit-rate encoding matrix.

It is yet another goal of the inventio=n to provide a technique of this kind thatis adapted to MISO or MIMO type mult—-antenna systems for s-ingle-carrier or multi-carrier ts/pe modulations combined wisth the different techniques of multiple access, namel y CDMA (Code Division Multiplex Access), FDMIA (frequency division multiple access) or TDMA (time div ision multiple access).

It is an other goal of the invention to propose a technique o f this kind that canbe used to augment the spatial diversity of the systems while at the same time

4 T- PTS

Fa0ns reducing imterference between the different transmission channels to te minimum and limitirg the loss of spectral efficiency.

In -other words, it is a goal of the imvention to provide a techmique of this } kind that can be implemented in a practical and low-cost manner <n a system implementing a large number of antennas. 4. Essential characteristics of the invention

Theese goals as well as others that sThall appear here below are achieved by means of a method for transmitting a digital signal via n transmit antennas, » being stri ctly greater than 2, in which »n source vectors to be transmitted respectively by each of said transmit antennas are associated with a source data vector by means of an encoding matrix _M of rate equal to 1, usirag reference symbols k=nown to at least one receiver ard enabling this receiver tom estimate at least threes transmission channels respec tively corresponding to ezach of said oo transmit amntennas. -- Cott

Ac cording to the invention, said reference symbols of a &ransmission method off this kind undergo a mathematical transformation by sa id encoding matrix M Before they are transmitted.

Th-us, the invention relies on an eratirely novel and inventive approach to the transmission of a digital signal, implermenting an encoding matrix in a multi- antenna sy~stem with more than two transm -t antennas.

Moore specifically, the invention proposes the transmissior, on the n transmit amtennas, of the reference symbols of the encoding matrix M =of rate equal to 1, a vector of reference symbols being associated with the encodimng matrix M by means «of an encoding function.

Suech an encoding matrix M of rate equal to 1 corresponds either to a non- orthogonal matrix or to a block orthogonal matrix, the rate being defined as the ratio betw=een the number of symbols transmitted and the number of symbols times duri-ng which they are transmitted.

Ad_vantageously, the reference symbols are distributed in space and in time and/or in sspace and in frequency.

Amended page as filed on 5" De=cember 2006

The encoding matrix then implements a space-time encoding and/or space- frequency encoding.

Accordirg to a first embodiment, the e=ncoding matrix includes at least two blocks, each of the blocks being orthogonal.

PreferabMy, each of the blocks of reference symbols is tmransmitted . separately, each of the blocks being transmit&ed on certain transmit antennas, the other transmit artennas being powered off.

Thus, thes data transmitted by a first sett of antennas are not disturbed by the data transmittecd by another set of antenn.as, the other set of antennas not transmitting on fhe same carriers at the same point in time,

Accordimg to another embodiment of the invention, called a third embodiment, th e transmission method comperises a step of selection Tbetween a frequency distrilbution and a time distribution.

In particular, this selection step may ta_ke account of the characteristics of a transmission channel.

Accordirag to another embodiment =of the invention, called a second embodiment, th-e reference symbols are transmitted on all the transmit antennas after mathematical transformation by the enco ding matrix M.

Thus, the encoding matrix M is a comporehensively non-orthogonaal matrix.

In particwilar, the encoding matrix M may be obtained by a Jafarkhani type encoding and hams the form:

HX x3 x4

M= : SI

TX TX Hx

Xo TX TH 4d where x_ is a reference symbol and x is a conjugate reference symbol with { being a re-lative integer, 1 < i < 4,

The inve ntion also relates to correspon ding transmit device.

As indiccated here above, the invention can thus be applied to uplink communicationss, a transmit device then cor—responding to a terminal (or being

Amended page= as filed on 5" December 2006 ’ John L. Spicer

Sb Patent Attorney included in a terminal), as well as to downlink communicat=ions, a transmit device corresponding, in this case, to a base station (or being inclucied in a base station).

The invention also relates to a digital signal formed by n vectors respectively transmitted by means of » transmit antennas, a» being strictly greater 5S than 2.

:

According to the imvention, the signal compris=ses encoded reference= symbols, obtained after mathematical transformation of reference symbols by am encoding matrix M of unitarzy rate, so as to enable the estirmation, in a receiver, off : at least three transmission channels respectively correspmonding to each of the= transmit antennas.

The invention also relates to method of estimati-on of the transmissiorm channels in a multi-antenna system implementing n transmit antennas, where n is strictly greater than 2, and at least one reception anterina. According to this method, n vectors to be transmitted respectively by each o=f said transmit antennas are associated with a vector of source data, by means of an encoding matrix M_ implementing reference symbols known to at least one receiver and enabling this receiver to estimate at least three transmission chhannels correspondings= respectively to each of the transmit antennas.

According to the invention, such an estimation method comprises a step off reception of a received refer ence vector, corresponding to a transmitted reference= vector obtained by the multiplication of reference sym bols by said encodings matrix M, and modified by> at least one transmission chhannel for each of the transmit antennas. For eachh of said reception antennas, the received references vector undergoes a mathematical transformation by a decoding matrix, which is the inverse of the encodimg matrix and takes accoumt of the effect of = transmission channel associaated with the reception antenn=a, to give an estimatiorm of the effects of the transmission channels on the reference symbols.

Thus, the invention relies on an entirely novel anc] inventive approach tos channel estimation in a multi-antenna system with meore than two transmi® antennas. It will be noted thaat this approach is also nove=l in a system with tw transmit antennas.

Indeed, the estimation of the different tran smission channels is implemented from reference symbols known to at least omne receiver, a vector off reference symbols being ass ociated with an encoding ma_trix M by means of ara encoding function.

With the vector of reference symbols and the encoding matrix M used being known, it is possible tO estimate the different transmission channels from the inverse of the encoding matrix, corresponding to the decoding matrix. - Thus, from reference symbols and the encoding technique used, a reception device may implement techniques of decoding, filtexring or equalization, and a recombination of the signals coming from the various antennas, in order to estimate the different transmis sion channels.

Advantageously, the decoding matrix is an inverse matrix integrating an equalization in the sense of the MMSE ("Minimum Mean Squared Error") or ZF ("Zero Forcing") criterion.

In particular, the criterion implemented may be the MCMSE criterion. The decoding matrix is then formed by the elements:

CR

MIM += 4 ’

The criterion implemented may also be the ZF criterion. The decoding matrix is then formed by the elements: pe MT,

MM with: - r being the received reference vector; - M the encoding matrix; - 1 the unitary matrix; - vy the signal-to-noise ratio; - H the conjugate transpose. :

Preferably, the estimation method comprises an interpolation step delivering an estimation of the transmission channels for each. of the payload data from the estimation of the reference symbols.

In particular, the interpolation step is noteworthy in that it implements a temporal interpolation and/or a frequency interpolation.

This interpolation step may belong to the group comprising: - linear interpolations;

Amended page as filed on 5" December 2006 ~ - Wien er interpolations.

Anothesr aspect of the invention relates to a reception devices in a multi- antenna systerm implementing » transmit antennas, where 7 is strictly = greater than 2, and at lea st one reception antenna, in which n vectors to be= transmitted respectively b-y each of said transmit antemnas are associated with a vector of source data, bZy means of an encoding matri= M, implementing refere=nce symbols known to saiad receiver and enabling this receiver to estimate n transmission channels corre=sponding respectively to each of said transmit antennas _

Such & reception device comprises means of reception off a received reference vect or, corresponding to a transmitted reference vector obtained by the multiplication of reference symbols by said encoding matrix M, and modified by at least one transmission channel for each eof the transmit antennas. For each of said reception antennas, the received reference vector undergoes a rmathematical transformatiorn by a decoding matrix, which is the inverse of the enceoding matrix and takes acc-ount of the effect of a trans mission channel associamed with the reception antemna, to give an estimation of the effects of the transmiss-ion channels on the referencce symbols.

As inclicated here above, the in—vention can be applied to uplink communicatio ns, the reception device then. corresponding to a basee station (or being includecd in a base station), as well as to downlink commun_ications, the reception devi ce corresponding in this case to a terminal (or being included in a terminal). 5. List of figures

Other features and advantages of the invention shall appear mmore clearly from the following description of a prefermred embodiment, given boy way of a simple, illustrative and non-exhaustive exam ple, and from the appendeed drawings, of which: - Figures 1A and 1B present a system of channel estMmation in a multi-antenna system with four transmit antennas, with symbols distributed in the frequency domain (figure 1A) or ttime domain (figure 1B) according to a first embodiment of the invemntion;

Amended page as filed on 5" December 20065 - Figures 2A and 2B present a particular distribution of the symbols of the chmannel estimation system of figures 1A and 1B; - Figures 3A and 3B illustrate a system of channel estimation in aa multi-antenna system with four transmit antennas, with symbols distributeed in the frequency domain (figure 3A) or time domaim ) (figure 3 B) according to a third embodiment of the invention. 6. Descripsion of an embodiment of the invention

The general principle of the invention relies ora the association of an_ encoding matrix M wi th a vector of reference symbols, known to at least so as to eraable the estimation, in the receiver, of the different propagation channels between more than two transmit antennas and a reception. antenna.

This encoding rmatrix M is either non-orthogonal or block orthogonal and has a rate equal to 1, the rate being defined as the ratio between the number of™ symbols transmitted amd the number of symbol times during which they are transmitted. The symbols of the encoding matrix M are then distributed in time- and/or in frequency on «ach of the transmit antennas.

At reception, fo r each reception antenna, the recei ved signal is multiplied by the inverse matrix (integrating an equalizing technique as understood according to the MMSE or ZF criterion) of the encoding matrix M, in taking account if necessary of the noise introduced by the receiver.

The result is a vector with n dimensions representing the n» transmission channels bestween the n transmit antennas and this reception antenna. “This vector with n dimmensions is then used by the meceiver to estimate the tr—ansmission : channels. This is done for example by repeating this operation perio«dically and performing a time and/or frequency interposlation between two referen ce symbols estimated during this operation. The interpolation is, for example, of ~ a linear or

Wiener typee.

Refeerring now to figures 1A and 1B, a description is givern of a first embodimernt of the invention in which it &s sought to estimate the transmission channels ofS a multi-antenna system with fouar transmit antennas.

Acc-ording to this first embodiment, the encoding matrix M is a block matrix, eacch of the blocks comprising n reference symbols. Arm Alamouti orthogonal space-time encoding is then agoplied to each block of th e encoding matrix M. Fach of the blocks of n reference symbols is then orthogonal _.

Tho se skilled in the art will easily extend this teaching to the case where the number of antennas, in transmission arad/or in reception, is greater. It is thus possible to apply an Alamouti encoding to each of the blocks of a systesm with n= 4, 6, 8,... transmit antennas.

Acc-ording to this embodiment ill ustrated in figures 1A arnd 1B, the Alamouti emncoding is applied to the refererace symbols used for the esstimation of the channel . Then, these encoded reference symbols are transmitted on one pair of antennas whhile the other pair of antennas is kept powered off.

Thus, if we consider the vector of reference symbols [x x, => x] the encoding matrix M which for its part is associated by means of th e encoding functionis: x x 0 0

IC 0

M - aN 0 0 -x, x;

where x, is a reference symbol, x; is a conjugate reference symbol, with i } as a relative integer and 1 < i < 4, and 0 signifies that no symbol —is transmitted on the concerne=d antenna. :

Each_ block A 2 and , o of the encodimmg matrix being xX x Xs 5 encoded according to an Alamouti code, we have MM = I, vith I as the unit matrix, and “** as the conjugate transpose.

The mreference symbols of the encodizng matrix M are then transmitted after space-frequesncy distribution ( (figure 1A) or space-time distribiation (figure 1B) on the diffemrent transmit antennas, the spaces axis representing thee columns of the matrix M anad the frequency axis (figure 1A) or time axis (figure 1B) representing the rows of sthe matrix M.

It is clear that other space-time or space-frequency disttributions of the symbols car be envisaged, as also a combination of the space—time and space- frequency dmistributions.

In fa ct, each block of the encoding rmatrix M is transmittesd independently on its respecctive antennas, while the other blocks of the encodirmg matrix are not transmitted. In other words, each block of reference symbol s is transmitted separately, e=ach of the blocks being transmitted on certain transmit antennas while the other anttennas are powered off.

Thuss, figure 1A presents the symbols transmitted by the our antennas 11, 12, 13, 14 cof a multi-antenna system with four transmit antenraas, the symbols transmitted being distributed in the frequeracy domain (y-axis) with X, being a reference symbol referenced 15, X; a conjugate reference symmbol (i a relative integer and 1<i<4), x a data symbol referenced 16, and 0 signifies that no symbol is transmitted.

The =symbols transmitted by the four antennas 11, 12, 13, M4 are distributed in the space—frequency domain (figure 1A) or space-time domaina (figure 1B) as a function of the parameters AF, Af}, Af; (figure 1A), AT, Aty, At, (figure 1B), representing the repetition patterns of the reference symbols.

The values chosen for Af, corresponding to the spacing between two reference carriers (in this example Af= {AF, Af}, Af}), and for At, corresponding to the spacing between two refererace symbols at known poinats in time (in this : example At = {AT, At), Aty}), are no t proper to the proposed sys tem but depend on the stationary state of the transmission channel.

In general, the following are assumed: - AF << Bg, with Bc th € coherence band of the cha _nnel; - AT << Tg, with Tc the coherence time of the charnel ; - Af, verifies at best the frequency stationary state eof the channel; - At; verifies at best thes temporal stationary state off the channel; - Af and At; depend on a compromise between the loss of spectral efficiency and the performance of the system.

Figures 2A and 2B also present another example eof the space-time distribution (figure 2A) or space-frequency distribution (figure 2B) of the symbols in this first embodiment, as a functi on of the parameters AF, AX, Af, (figure 24),

AT, Aty, At; (figure 2B).

In this example, the values of the reference symbols =are chosen so that

X,=X,and X, = X,.

The reference vector receive d at the level of a reception antenna, modified by -the transmission channel, can then be written in the form r = Xk +n, where A corresponds to a modeling of the transmission channel and » iss a Gaussian white noi se vector.

This received reference vector can also be written in vector form: n x x 0 0h n, ; : H : : fin r, 0 0 x; xh] |n r 0 0 -x, x lh] Ln,

For each of the reception antennas, it is sought to estima_te the transmission channel # by applying, to the received reference vector, a mathematical transforma-tion by a decoding matrix, corresponding to tle inverse matrix integrating: a technique of equalization, in the sense of the MMSSE or ZF criterion, of the encomding matrix M. . According to the MMSE criterion, the decoding matrix is formed by the elements:

A MH h= —"

MM += 4 with : - r as said received reference vector; - M said encoding matrix; - I a unitary matrix; - y the signal-to-noise ratio; - 7 the conjugate transpose.

According to the ZF criterion, the decoding matrix is formed by the elements:

H h= 2,

MM with: - r as said received reference vector; - M said encoding matrix ; - # the conjugate transpose.

These two criteria lead to identical results with a high sigznal-to-noise ratio.

In tThe case of a ZF criterion, we Obtain:

Ly LI kh, a 1 a 0 0 Ohl |“ 1 a — 0 — De bi 7° 0 a 0 0h On hl lo o L of0 O08 0lkilg o L

A, blo 0 0 bla 5 0 0 0 — 0 0 O= -— b b , 2 4 2 withh a = EA and b= lx] . i=l i=3

It iss thus possible to determine thie coefficients of the channel at an instant p on a carrier k, at an instant p on a carrier k+Afj, ..., as illustrasted in figure 2A.

a Pah E02

By applying a frequency interpeolation between the two carriers k and k+Af) bearing the reference symbols, thee receiver can determine the coefficients of the prropagation channel at the carriers k, k+1, k+2, ..., k+Afj-1, k=+Af;. : -An interpolation can also be mad. ¢ in the time domain, in considering that the refesrence symbols are transmitted at t-he instant p on the carrier k=, at the instant p+At or the same carrier k, .... The receiver can then determine the coefficients of the propagation channel at the instants go, p+1, p+2, ..., p+At-1, p—+At and so on and so forth. “The receiver can therefore perform a time interpolation and/-or a frequency interpolation. This interpolation step implements an interpolation ®echnique well known to those skilled in the art, such as for example a linear type imnterpolation or a Wieneer interpolation.

Since the other pair of antennas does not transmit on the same carriers and at the same instants, as illustrated in figures 1A, 1B, 2A and 2B, the signal transmi tted by the first pair of antennas iss not disturbed.

Each pair of antennas then alternately transmits the refer-ence symbols distributed on its antennas, so as to estimate all the transmission channels of the multi-amtenna system. -According to the invention, it is thus possible to apply orthogonal space- time co des to systems having a greater m umber of transmit antennas, by means of an encoeding matrix M preserving a rate equal to 1. It is thus possible to apply an

Alamotati code with a rate equal to 1 to systems having 4, 6, &, ..., transmit antennas (whereas in the prior art, thme number of transmit ammtennas of the transmi ssion system is limited owing to tthe use of orthogonal space—time codes).

However, although this channel estimation technique perfeorms better in terms o f estimation, since the other groups of antennas are powered off when one group Of antennas is transmitting, this technique is accompanied by a loss of spectral efficiency and does not benefit fom the total power of the antennas since certain carriers convey no information at defined instants. -A second embodiment of the iravention is then presented, wherein the reference symbols are transmitted on all the transmit antennas after— mathematical transformation by the encoding matrix M, the eencoding matrix M beimng non- orthogonal.

According to the second embodiment, a Jafarkhani type non-ortThogonal . space-time encoding, zs presented in "A Quasi—Orthogonal Space-Timee Block 5S Code" (IEEE Transactions on Communications, Vol. 49, N°1, 2001, pp .1-4), is applied to the reference= symbols used for the estimation of the channel.

This encoding is used especially to ®ransmit signals showing low interference.

Thus, if we consider the vector of reference symbols [x x, x, x], the 190 encoding matrix M assOciated with it by means of the encoding function is : noon xXx xX; —Xx, IX X

Xo xX XH ox where x, is a reference symbol, x, is a co njugate reference symbol] with { as a relative integer, | << i< 4.

All the reference symbols of the encoding matrix M are then tramsmitted after space/frequency cistribution on all the trarsmit antennas, the spatial axis representing the colurmns of the matrix M an d the frequency or tizme axis representing the rows o fthe matrix M.

As described h_ere above, the reference vector received at a reeception antenna, modified by the transmission channel, can be written in tke form 2€0 r= Xh+n, where h co-rresponds to a modeling of the transmission chann el and n is a Gaussian white noi=se vector.

This received re=ference vector can also be =written in vector form: i x 2 *3 || A n, i : : : < Hi nt |-x -xi x xh] {nl ry X, =X -x, x || A n,

Once agaim, for each of the reception antennas, it is sought to estimate the transmission charnel # by the application, to the received reference vector, of a mathematical trarasformation by a decoding matrix_, corresponding to the in_verse . matrix integratingz a technique of equalization in the sense of the MMSE or ZF criterion of the en_coding matrix M.

In the cases of an MMSE criterion, we get: 1 B 1 B a+— 0 0 b a+-— 0 0 b

A Y Y hy 1 a 0 0 bh 1 i 0 at— ~b 0 [lo o_o ola 0 a+— -b 0 2= 4 1 2 | y 1 M%.n

Blo -b a+= oo ||0 0 a O}hy | o _, 41 h, 4 b 0 0 ala, 4 b 0 0 a+ 1 b 0 0 a+ La ¥ r 4 2 with a= |x,| and b=2Re(xx; —x,x;). i=]

This opemration is reiterated identically for each reception ant—enna, whatever the number of antennas. It is thus possible to determine the coeffiecients h, of a transmission channel at a frequency c or at a defined instant c, and aH] that remains to be domme is to apply a time and/or frequeracy interpolation at the receiver between the estimates of kh, and #4, , (with k = Af sThould ¢ be a frequency amd k =

At should ¢ be an instant) in order to assess the missing values. Thus, comprehensive kmowledge is obtained of all the values of a transmission chmannel for each of the artennas, thus making it possible teo equalize the reception =signal conventionally.

Accordingz to this second embodiment, Et is possible to exten this estimation technique to systems having more than t=wo transmit antennas.

Thus, if we consider the vector of reference symbols fx, x, ... x] the encoding matrix 7 that is associated with it by meas of the encoding function is: xX ee. x

M=| : RE

Xone] oe xy where x; is a reference symbol, with i as a relative integer 1 < i< N, and } N =n?

This is a full-ranking matrix sO that it can be inverted durimg the estimation - of the dif erent channels.

A s described here above, the reference symbols of the en«coding matrix M are transrmitted after space/frequency distribution on all the transrmit antennas, and the referesnce vector received at a reception antenna, modified by the transmission channel, «can be written in the form: ; : x k :

Bl_|¥s % % X CANE iE Xo Xo Xy X,|| hy) | ns rn Xa xX, Xs Xgllhd Ln,

Ornce again, for each of the reception antennas, the re=ceived reference vector re=ceives the application of a mathematical transformation by means of a decoding matrix, corresponding to thie inverse matrix integrating a technique of equalizat ion in the sense of the MMSE or ZF criterion of the en coding matrix M in order to estimate the transmission channel 4.

If an equalization technique in_ the sense of the MMSE criterion is used, we get: ; | i

Fa |_ pened] ok fps] Apt,» With y the signal-to-noise ratio.

Fa, y h, y

A, hy

A_s described here above, it is then possible to determmine the missing coefficiemts of the transmission channel by applying a tinme or frequency interpola-tion (or both) to the receiver in using a classic technique of interpolation.

R_eferring to figures 3A and 3B, we now present a third ernbodiment of the inventiora that can be applied more particularly to MIMO typee multi-antennas systems.

Ira this third embodiment, a flexible principle is proposed for the application of either a space-time encoding or a space-freqguency encoding,

depending on the characteristics of the transmission channel.

Thus, figures 3A illustrates the transmission off four reference symbols ard their conjugates temporally spaced out, in a mult i-antenna system with fowur . transmit antennas with X; being a reference symbol referenced 15, X; a conjugate reference symbol (# a relative integer and 1< i < 4), x a data symbol, referenced 16.

Figure 3B illustrates the transmission of four reference symbols and thesir conjugates spaced out frequentially, in a multi-antenmna system with four transmit antennas.

In this third embodiment, the reference symbols, once encoded by meamns of the encoding matrix M, are distributed along the time axis or frequency axis according to the properties of the propagation channel.

It is then possible to switch from a space-time encoding to a space- frequency encodings.

It may be recalled that the values chosen for Af (spacing between tvwo reference carriers) and At (spacing between two reference symbols at knoven instants) are not proper to the proposed system but: depend respectively on tlhe band and time of coherence of the transmission chanmel.

As a general rule, the distribution in the time domain is applied rather in the case of the chamnel that varies temporally while the frequency distribution is applied more to a channel that varies frequentially.

Thus, with a priori knowledge of the chanmel or having computed the values of the coherence band or the coherence time of the channel, it is possible to switch between the two structures of insertion of mreference symbols describ -ed here above.

Those skilled in the art will easily extend the teaching of these three embodiments to sy/stems having a greater number of antennas, as well as to systems having a space-time distribution and/or a space-frequency distribution different from those proposed in figures 1A, 1B, 2A, 2B, 3A and 3B.

Thus, according to the invention, the differen. t transmit antennas transmit, on a same carrier and at a same instant, a signal characterized by a space-tirme emcoding and/or space-frequaency encoding, thus limiting the loss of spectral exfficiency.

This signal therefore intrinsically comprises thee characteristics of the irvention.

Finally, a receiver may estimate each of the transmission channels between tie different transmit and reception antennas on the basis of this specific encoding: amd of the appropriate processing described here above=. The particular channel esstimation technique proposed according to the invemntion may therefore bes agpplied in the case of a systerm having two transmit antenras.

Claims (21)

ae Come T3502 a Bm CLAIMS:

1. Method for transmitting a «digital signal via n transmi_t antennas, n being strictly greater than 2, in which n source vectors to be transmitted respectively by each of said transmit antesnnas are associated with a source data vector by means of an encoding matrix M of rate equal to 1, usming reference symbols known to at least one receiver amnd enabling this receiver %o estimate n transmission channels respectively corresponding to each of said transmit antennas , characterized in that said reference symbols undergo a wmathematical transfornnation by said encoding matrix M before they are transmitted.

2. Method of transmission accordingz to claim 1, characterized in that said reference symbols are distributed in space and in time.

3. Method of transmission accordi ng to one of the claimms 1 and 2, characterized in that that said reference symbols are distributed in space and in frequency.

4. Method of transmission accordmng to one of the claims 1 to 3, characterized in that said encoding matri= includes at least two blocks, each of said blocks being orthogonal.

5. Method of transmission according to claim 4, characterized ir that each of said blocks of reference symbols is transmitted separately, each of said blocks being transmitted on certain of said transmit antennas, the other transmit antennas being powered off.

6. Method of transmission accordmng to one of the claixns 1 to 5, characterized in that it comprises a steep of selection between a frequency distributi on and a time distribution.

7. Method of transmission according= to claim 6, characterized in that said selection step takes account of the characteristics of a transmission chmannel.

8. Method of transmission accord@ng to one of the clairns 1 to 7, characterized in that said reference symb>ols are transmitted on all the transmit antennas after mathematical transformation by said encoding matrix 4.

9, Method of transmission acceording to claim 8, characterized in that said encoding matrix is a matrix obtainezd by a Jafarkhani type en_coding and has the formm: : x x XX X= TX XX Xa TH TX A] where x, is a reference symbol and x; is a conjugate reference symbol with i bei ng a relative integer, 1 < i< 4,

10. Digital signal formed by n vectors respectively transm-itted by means of n trarsmit antennas, n being strictly greater than 2, chaaracterized in that the signal cormprises encoded reference= symbols, obtained after mathematical transformation of reference symbols by an encoding matrix M of mrate equal to 1, so as to enable the estimation, In a receiver, of said n transmit channels resypectively corresponding to each o-fsaid transmit antennas.

11. Method of estimation of the transmit channels in a multi-antenna system impplementing » transmit antennas, where »n is strictly greater than 2, and at least one reception antenna, according to “which rn vectors to be transmitted respectively by each of said transmit antennas ame associated with a vectomr of source data, by me ans of an encoding matrix M, immplementing reference symbols known to at lea:st one receiver and enabling thmis receiver to estimate ra transmit channels corresponding respectively to each o»fsaid transmit antennas, characterized in that this method comprises a step of reception of a received refeerence vector, corresponding to aa transmitted reference vector obtained by the multiplication of reference symbols by said encoding matrix ZV, and modified by at Least one transmission channel for- each of said transmit antemnnas, and in that, for each of said reception antennas, said receiv-ed reference vector unclergoes a mathematical transforamation by a decoding m_atrix, which is the inverse of said encoding matrix andl takes account of the effect of a transmission channel associated with said reception antenna, to give an estirmation of the effects of said tmransmit channels on said referesnce symbols.

12. Nlethod of estimation accordirg to claim 11, characterized in that said decodingz matrix is an inverse matrix imtegrating an equalization fin the sense of the MMSE or ZF criterion.

13. Method of estimation according to claim 12, charactemrized in that said criterion. implemented is the MMSE criterion and said decodingz matrix is formed by the eMlements: eM, MIM +— 4 > with: - r being the received reference vector; - M the encoding matrix; - I a unitary matrix; - y the signal-to-noise rati_o; - H the conjugate transpose.

14. Method of estimation accordimg to claim 12, charactemrized in that said criteriona implemented is the ZF criterion and in that said decoding matrix is formed boy the elements: Fe MT, MM with: - r being said received reference vector; - M said encoding matrix 5 - H the conjugate transposse.

15. Method of estimation according to any one of the «claims 11 to 14, characte=rized in that it comprises an iraterpolation step deliverin_g an estimation of said trarasmission channels, for each Of the payload data, from the estimation of the refer—ence symbols.

16. Method of estimation accordimg to claim 15, characte rized in that said interpolation step implements a temporal interpolation anclor a frequency interpolation.

Amended page as filed on 5" D®ecember 2006 —

17. Method of estimation according to army one of the claims 1 5 and 16, characterized in tThat said interpolation step belo=ngs to the group compri sing: - linear ingerpolations; - Wiener i nterpolations.

18. Device for transmitting a digital signal feeding » transmit antennas, » . being strictly greater than 2, comprising means for associating n source= vectors to be transmitted re spectively by each of said transmit antennas with a source data vector by means of an encoding matrix M o=f rate equal to 1, usings reference symbols known to at least one receiver and erabling this receiver to estimate n transmission channels respectively corresponding to each of said transmit antennas, characterized in that the device comprises means for the application of a mathematical cormversion to said reference symt>ols by said encoding matrix M.

19. Transmisssion device according to claimm 18, characterized in th at it forms or is integrated into at least one of the installations belonging to the group comprising: - abase= station; - atermminal.

20. Reception device in a multi-antenna =system, implementing = transmit antennas, where 7 is strictly greater than 2, anc] at least one reception antenna in which »n vectors to be transmitted respectively by each of said transmat antennas are associated wi th a vector of source data, by— means of an encoding matrix M, implementing refeerence symbols known to said receiver and enabling thmis receiver to estimate n trans=smit channels corresponding re=spectively to each of samd transmit antennas, characterized in that it comprises means of r—eception of a received reference vector, corresponding to a transmitted ref erence vector obtainead by the multiplication of reference symbols by said encoding matrix M, and m_odified by at least one transmission channel for each of said transmit antennas, and in that, for each of said reception antennas, said received referemnce vector undergoes a mat_hematical transformation by a decoding matrix, whaich is the

’ Amended page as filed on 5 "December 2006 - John L. Spicer 23 Patent Attorney inverse of the encoding matrix and takes ac-count of the effect of a transmission channel associated with said reception antenraa, to give an estimation of the effects of said transmi~t channels on said reference sy~mbols.

21. Recepti on device according to claim 220, characterized in that it forms or is : 5 integrated into at least one of the installations belonging to the group ~comprising: . - a baxse station; - ate:rminal.